Power efficient optical links are useful for a number of applications such as high-bandwidth free-space communications. The most efficient optical communications links utilize large-alphabet orthogonal modulation formats such as pulse-position modulation or frequency-shift keying (FSK) modulation. For FSK modulation, a transmitter transmits k bits of information by sending one of M possible frequencies during each symbol period where M=2k. The receiver determines which one of the M frequencies was transmitted in order to receive the k bits of information.
FIG. 1 shows a generic configuration for an FSK transmitter 10. The transmitter 10 includes a transmitter source 14, an FSK modulator 18, an optical power amplifier 22 and transmit optics 26. The FSK modulator 18 is used to select one of M optical frequency components (i.e., “tones”) in the optical signal generated by the transmitter source 14. The power amplifier 22 increases the optical power of the signal transmitted by the FSK modulator 18 and the transmit optics 26 are used to condition the optical signal for transmission to one or more receivers. For example, the transmit optics 26 can include optical components to achieve a desired beam geometry for the FSK-modulated optical signal for free-space transmission or for launching into an optical fiber.
There are several techniques that have been demonstrated for transmission of optical FSK waveforms. As shown in FIG. 2, a tunable laser 30 can be used as the combination of the transmitter source 14 and FSK modulator 18 of FIG. 1. The tunable laser 30 is tuned to a single tone by varying a bias current or by changing the characteristics of the laser cavity. The illustrated configuration 10′ is typically limited to symbol rates of a few GHz or less. An alternative configuration 10″ is shown in FIG. 3 and includes a number M of source lasers 32A to 32M (generally 32) where each source laser 32 operates on a unique tone. Each source laser 32 is intensity modulated by direct modulation of the laser current or, as shown, using an external modulator 34A to 34M (generally 34). The external modulators 34 are activated by control signals so that an optical signal from only one source laser 30 is transmitted during a symbol period. The optical signals exiting the external modulators 34 are combined along a single optical path by a combiner 36 (e.g., a wavelength division multiplexing (WDM) combiner) although during normal operation only one of the external modulators 34 permits its optical signal to be transmitted to the combiner 36.
While the transmitter configuration 10″ illustrated in FIG. 3 is useful at high symbol rates (e.g., rates exceeding 40 GHz), the complexity of the transmitter is impractical for a large number M of source lasers 32. For example, if the number M of source lasers 32 in the transmitter is 1,024, the number of external modulators 34 required is 1,024.